Hydraulic cements, such as portland cement and blended hydraulic cements, consisting of portland cement and pozzolans, have, for many years, been used as an essential ingredient of the mortar and concrete used for construction of structures, highways, etc. In many instances, fly ash has been used as the pozzolanic component in blended hydraulic cements, and as a mineral admixture to replace part of the more expensive portland cement used in mortar or concrete. The use of fly ash, in place of part of the portland cement or as a constituent in blended cements reduces the large amount of energy required to produce the hydraulic cements required for mortar and concrete. However, the prior art has taught that no more than 35 to 45 per cent of the portland cement can be replaced with fly ash. According to the teachings of the prior art, when more fly ash than that is employed, the resulting mortars and concretes do not have sufficient strength.
Fly ash, particularly Class C fly ash, is a waste material. Many million of pounds of this material are produced each year and must be disposed of. Because this fly ash contains potentially dangerous heavy metals, disposal is becoming a more and more significant problem. While merely adding water to the waste fly ash will cause it to harden, as is true of many solid materials, the hardened product has no significant strength, and the disposal problem is merely changed, without being solved.
Because of the significant waste disposal problem, uses have long been sought for this fly ash, but these uses have generally involved merely hardening the fly ash, as indicated above, an operation which does not provide a material with sufficient strength for any useful purpose.
Though the fly ash can be blended with portland cement to create a useful product, as indicated above, the amounts which can be used are limited, so that the massive amounts of this waste material available are not consumed. Also as indicated, merely hardening the waste material does not really provide for its effective utilization. To avoid the substantial costs of energy involved in producing cement, generally between $3 and $11 per ton, at current rates, if the fly ash, where there is no energy cost, because it is a waste material, could be used for structural uses, an extremely desirable situation would be created. This desirable situation is not accomplished through a mere hardening of the materials, as described, for example, in U.S. Pat. No. 4,101,332, Nicholson and U.S. Pat. No. 4,432,280, Kneller, et al.
It now has been found, however, according to the present invention, that a hydraulic cement, where all of the portland cement or blended hydraulic cement is replaced with fly ash, can be formed by activating a Class C fly ash with an alkali metal material, along with citric acid. Through employment of the composition of the present invention, a waste material, Class C fly ash, with relatively small amounts of activators and other conditioning agents, forms a useful, structural cement without the utilization of the energy otherwise required for such materials.
Not only is the fly ash employed for production of a usable, structural material, without the expenditure of significant energy costs, but, in addition, the costs which would otherwise be attributed to the disposal of the fly ash are avoided.